The natural radiation in space is caused by electrons and protons that are trapped by the magnetic field of the earth. The trapped protons occupy a belt that ranges from 1 to about 3.8 earth radii (when measured at the equator). The electrons are trapped in an inner zone that ranges from 1 to about 2.8 earth radii and an outer zone that ranges from about 2.8 to about 12 earth radii, one earth radius being about 4000 miles.
In space applications, integrated circuits are dependent to a large extent on external component shielding to protect the components from the bombardment of this natural radiation. External component shielding consists of satellite material, printed circuit boards, and any other material that make up a space craft. When necessary, specially designed boxes constructed of selected shielding materials are also employed.
Although, approximately 80% of the integrated circuits that are sold and produced today are packaged or encapsulated in epoxy, heretofore, plastic encapsulated packages have not been used successfully in space applications. The primary reason for their lack of success is because moisture easily penetrates through plastic packages which can cause dendritic growth and corrosion which consequently may result in electrical shorts and open circuit conditions. Secondly, the accumulation of radiation eventually causes them to fail. Although, plastic packages can be used in space (providing that sufficient outer shielding is also included), the excess outer shielding does add a considerable amount of weight.
While it is known in the art to add thermally conductive heat sinks to the die attach pad in a plastic package, the inventors are not aware of any method of molding an upper shield into a plastic package because of the difficulty in securely positioning the shields (i.e., the possibility of an upper shield moving during the molding process could easily short to the die and wirebonds). Even if it were possible to mold shields into the package, shields would have to be customized for each die size and package combination. Given the low volume of components needed, this approach would be extremely costly.
Thus, the state-of-the-art for providing environmental protection and total dose radiation shielding of an integrated circuit die at the package level consists of modifying existing alumina ceramic package designs or designing new alumina ceramic packages to incorporate copper tungsten shield plates. This is achieved by first cofiring a copper tungsten lower plate as an integral part of a cofired alumina ceramic package. The lid of the package then forms the upper shield. The exact location, thickness and material used for the shielding plates are critical factors which determine the shielding effectiveness of a given device.
What is needed is a shielded package which is light-weight, low cost, which can be manufactured quickly and inexpensively, and which provides protection from the environment, as well as, total dose radiation shielding of the integrated circuit die.